This invention relates to a thermoelectric material and a thermoelectric element and, more particularly, to a thermoelectric material in chalcogen series containing transition metal such as titanium and a thermoelectric converting element using the same.
Alternative energy from fossil fuel and environmental conservation are important in the world, and are discussed worldwide. Thermoelectric converting technologies are attractive alternative energy sources. Thermoelectric conversion does not produce any anti-environmental gas such as carbon dioxide gas and nitrooxides. Waste heat is reused as electric energy through the thermoelectric conversion, and refrigerants adverse to the environment such as freon are not required for refrigerators. Thus, thermoelectric conversion is free from environmental destruction, and thermoelectric conversion technologies are promising.
The efficiency of the thermoelectric conversion is defined by the xe2x80x9cfigure of meritxe2x80x9d ZT, and the figure of merit is expressed as
ZT=S2T/xcfx81xcexaxe2x80x83xe2x80x83(1)
where S is Seebeck coefficient, T is temperature, xcfx81 is the electrical resistivity and xcexa is the thermal conductivity. When the figure of merit increased in value, the thermoelectric converting efficiency is improved. From equation (1), it is understood that a large figure of merit is achieved by a substance which has a large Seebeck coefficient, small electrical resistivity and small thermal conductivity. The figure of merit has a certain temperature dependency unique to the thermoelectric material. For this reason, the thermoelectric materials have each temperature ranges available for applications.
Electric generators driven by steam turbines and compressor-type refrigerators are now the major thermoelectric converter. In order to achieve the thermoelectric efficiency larger than that of the electric generators/refrigerators by using the thermoelectric materials, the thermoelectric materials are to have the figure of merit ZT of the order of 3.
Even though such a large figure of merit ZT is difficult, certain thermoelectric materials are active in the relatively low temperature range, in which the steam turbines can not operate, and have a position superior to the conventional thermoelectric converters. Nevertheless, the thermoelectric materials are to exhibit the figure of merit greater than 1 for practical application.
Various non-oxide semiconductor thermoelectric materials have been proposed. Thermoelectric materials in the bismuth-tellurium series have the unique temperature range from room temperature to 400 degrees in centigrade, and exhibit good performance in the unique temperature range. Thermoelectric materials in the lead-tellurium series exhibit good performance until 700 degrees in centigrade, and silicon-germanium series exhibit good performance until 1000 degrees in centigrade. The thermoelectric materials in the bismuth-tellurium series are used for the thermoelectric refrigeration, and the thermoelectric materials in the lead-tellurium series and silicon-germanium series are used for the thermoelectric generation.
On the other hand, (Zn0.98Al0.02)O, AB2O4, which is disclosed in Japanese Patent Application laid-open No. 7-231122, and NaCo2O4 are examples of the oxide thermoelectric material. A and B are metal elements, and In is contained in B site. (Zn0.98Al0.02)O, AB2O4 and NaCo2O4 are proposed as thermoelectric generators using a high temperature heat source of the order of 700 degrees in centigrade and thermoelectric generators active from room temperature to high temperature range. However, those thermoelectric materials exhibit the figure of merit of the order of 1.
A problem inherent in the conventional thermoelectric materials is the low conversion efficiency. The conventional energy converters are undesirable from the viewpoint of the environment and safety. The petroleum, coal, natural gas and electric power generated by the nuclear power plants will be superceded by clean energy obtained through the thermoelectric conversion. If so, there will be a great demand for thermoelectric material greater in converting efficiency than the conventional thermoelectric materials in the bismuth-tellurium series, lead-tellurium series and silicon-germanium series.
It is therefore an important object of the present invention to provide thermoelectric material, which exhibits the thermoelectric converting efficiency greater than the conventional thermoelectric materials.
It is also an important object of the present invention to provide a thermoelectric generator, which achieves a high energy converting efficiency.
It is also an important object of the present invention to provide a thermoelectric refrigerator, which achieves a high energy converting efficiency.
In accordance with one aspect of the present invention, there is provided a compound having a CdI2 analogous layer structure and expressed by general formula of AxBC2xe2x88x92y where x is fallen within the range of 0xe2x89xa6xxe2x89xa62 and y is fallen within the range of 0xe2x89xa6y less than 1, the A is at least one element selected from the group consisting of lithium, sodium, potassium, rubidium and cesium, the B is at least one element selected from the group consisting of titanium, vanadium, chromium, zirconium, niobium, molybdenum, hafnium, tantalum, tungsten, iridium and tin, and the C is at least one element selected from the group consisting of sulfur, selenium and tellurium.
In accordance with another aspect of the present invention, there is provided a compound having a CdI2 analogous layer structure and expressed by general formula of AxBC2xe2x88x92y where x is fallen within the range of 0xe2x89xa6xxe2x89xa62 and y is fallen within the range of 0xe2x89xa6y  less than 1, the A is vacant, the B is at least one element selected from the group consisting of titanium, vanadium, chromium, zirconium, niobium, molybdenum, hafnium, tantalum, tungsten, iridium and tin, and the C is at least one element selected from the group consisting of sulfur, selenium and tellurium.
In accordance with yet another aspect of the present invention, there is provided a compound having a CdI2 analogous layer structure and expressed by general formula of AxBC2xe2x88x92y where x is fallen within the range of 0xe2x89xa6xxe2x89xa62 and y is fallen within the range of 0xe2x89xa6y less than 1, the A is at least one element selected from the group consisting of lithium, sodium, potassium, rubidium, cesium, magnesium, calcium, strontium, barium, titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, zinc (Zn), zirconium, niobium, molybdenum, ruthenium, rhodium, palladium, silver, cadmium, hafnium, tantalum, tungsten, rhenium, iridium, platinum, gold, rare-earth elements containing scandium and yttrium, boron, aluminum, gallium, indium, thallium, tin, lead, antimony and bismuth, the B is at least one element selected from the group consisting of titanium, vanadium, chromium, zirconium, niobium, molybdenum, hafnium, tantalum, tungsten, iridium and tin, and the C is at least one element selected from the group consisting of sulfur, selenium and tellurium.
In accordance with still another aspect of the present invention, there is provided a thermoelectric generator comprising a first piece of thermoelectric material and a second piece of thermoelectric material different from the second thermoelectric material, the thermoelectric material having a CdI2 analogous layer structure and expressed by general formula of AxBC2xe2x88x92y where x is fallen within the range of 0xe2x89xa6xxe2x89xa62 and y is fallen within the range of 0xe2x89xa6y less than 1, in which the A is at least one element selected from the group consisting of lithium, sodium, potassium, rubidium and cesium, and in which the B is at least one element selected from the group consisting of titanium, vanadium, chromium, zirconium, niobium, molybdenum, hafnium, tantalum, tungsten, iridium and tin, and in which the C is at least one element selected from the group consisting of sulfur, selenium and tellurium.
In accordance with yet another aspect of the present invention, there is provided a thermoelectric refrigerator comprising a first piece of thermoelectric material and a second piece of thermoelectric material different from the second thermoelectric material, the thermoelectric material having a CdI2 analogous layer structure and expressed by general formula of AxBC2xe2x88x92y where x is fallen within the range of 0xe2x89xa6xxe2x89xa62 and y is fallen within the range of 0xe2x89xa6y less than 1, in which the A is at least one element selected from the group consisting of lithium, sodium, potassium, rubidium and cesium, and in which the B is at least one element selected from the group consisting of titanium, vanadium, chromium, zirconium, niobium, molybdenum, hafnium, tantalum, tungsten, iridium and tin, and in which the C is at least one element selected from the group consisting of sulfur, selenium and tellurium.
Thermoelectric Material
The inventors discovered that attractive thermoelectric materials, which had a CdI2 analogous layer structure obtained through an appropriate element substitution, intercalation and desulfurization, exhibited the electrical resistivity of the order of 1 mxcexa9cm and Seebeck coefficient larger than 100 xcexcV/K. The thermoelectric materials were expressed by general formula (2).
AxTiS2xe2x88x92yxe2x80x83xe2x80x83(2)
The A-site was occupied by at least one Group 1 element, i.e., lithium (Li), sodium (Na), potassium (K), rubidium (Rb) and cesium (Cs). Otherwise, the A site was a vacant site, and, accordingly, the thermoelectric material was expressed as (TiS2xe2x88x92y).
The inventors further discovered that the following compounds exhibited good thermoelectric properties. The compounds were expressed by the general formulae, in which part of or all of the Group 1 element or elements in the general formula (2) was replaced with at least one of the elements selected from the group consisting of Group 2 elements, i.e., magnesium (Mg), calcium (Ca), strontium (Sr) and barium (Ba), 3d-transition metals from titanium (Ti) to zinc (Zn), 4d-transition metals from zirconium (Zr) to cadmium (Cd), 5d-transition metals from hafnium (Hf) to gold (Au), rare-earth elements containing scandium (Sc) and yttrium (Y), boron (B), aluminum (Al), gallium (Ga), indium (In), thallium (Tl), tin (Sn), lead (Pb), antimony (Sb) and bismuth (Hi).
The compounds were further expressed by the general formulae, in which part of or all of titanium (Ti) is replaced with at least one element selected from the oup consisting of vanadium (V), chromium (Cr), zirconium (Zr), niobium (Nb), molybden (Mo), hafnium (Hf), tantalum (Ta), tungsten (W), iridium (k) and tin (Sn).
The compounds were further expressed by the general formulae, in which part of or all of sulfur (S) was replaced with at least one element selected from the group consisting of selenium (Se) and tellurium (Te).
The thermoelectric materials exhibited high energy converting efficiency, and the problem is solved.
The low electrical resistivity is derived from the fact that the CdI2 analogous layer structure is a laminate of TiS6, which exhibits a relatively high electric conductivity. The TiS6 layers are bonded by the weak van der Waals force, and, accordingly, the thermal conductivity is relatively low. Moreover, the thermal conductivity is further reduced in the presence of the element or elements intercalated into the boundaries between the layers. The compound with the CdI2 analogous layer structure has the large Seebeck coefficient by virtue of various features such as the large degree of freedom of orbital, electron correlation and low dimensionality.
The compounds with the CdI2 analogous layer structure which were expressed by the general formulae of AxTiS2xe2x88x92y where 0xe2x89xa6xxe2x89xa62 and 0xe2x89xa6y less than 1 have layers of hexagonal dense packing of sulfur ions, and Ti atoms occupy octahedral sites in every other sulfur layer. In other words, edge-sharing TiS6 octahedra form infinite layers, and these layers are stacked. The TiS6 layers are bonded to each other by the weak van der Waals force, and various kinds of ions and molecules are intercalated into the van der Waals gap between the TiS6 layers.
The general formulae are represented by the sulfur-deficient chemical composition. The sulfur deficiency is categorized into two groups. In one group, sulfur atoms are removed from the TiS6 layers. In the other group, excess Ti atoms introduced between the TiS6 layers. Electric-conductivity-carriers are introduced by the deficiency of anion such as sulfur, element substitution by different valence elements, and the intercalation of atoms or molecules in the van der Waals gap between the TiS6 layers. In the TiS6 layers, the orbitals of cations such as Ti atoms overlap directly or indirectly overlap through the p-orbitals of the anions, thus, the electrical resistivity is low.
There is not any limit on a method for introducing the carrier. One of the carrier introduction methods is to make the sulfur atoms deficient through a quenching from a high temperature or a heat treatment in reduction atmosphere created by using hydrogen gas, nitrogen gas or argon gas. Another carrier introduction method is to substitute an element with different valance elements. Yet another carrier introduction method is to intercalate atoms and/or molecules into the van der Waals gap between the TiS6 layers. The above-described methods may be mixed or combined.
In case where the introduction of carrier is carried out by intercalating atoms into the A-site, it is preferable that at least one of the above-described elements at the A-site ranges from 0.1 less than x less than 0.5. In case where the introduction of carrier is carried out by substituting the above-described element or elements except for Ti for the original element at the B-site, it is preferable that the element or elements are fallen within the range from 0.1 mole % to 10 mole % in all the elements at the B-site. In other words, at least one of the elements to occupy the B-site except Ti ranges from 0.1 mole % to 10 mole %, and the remaining B-site is to be occupied by Ti.
In case where the introduction of carrier is carried out by substituting the above-described element or elements except sulfur for the original element at the C-site, it is preferable that the element or elements are fallen within the range from 0.1 mole % to 10 mole % in all the elements at the C-site. In other words, at least one of the elements to occupy the C-site except sulfur ranges from 0.1 mole % to 10 mole %, and the remaining C-site is occupied by sulfur.
Since the TiS6 layers are bonded by the weak van der Waals force, the electric conduction shows strong two-dimensionally nature, and Seebeck coefficient is enhanced by such a low dimensional electronic state. The orbital degeneracy takes place in the d-orbital of the cation such as Ti and the merged orbits in the octahedral structure of TiS6, and the entropy of the doped carrier is large. For this reason, the compounds exhibit the large Seebeck coefficient. Moreover, the cations such as Ti form a triangular lattices in the network where the ridgelines are shared among the octahedrons of TiS6, and the geometrical spin, orbital frustration and other electron correlation are strongly influential. This results in increase of the Seebeck coefficient.
Phonon and carrier are participated in the heat conduction. It is important to restrict the heat conduction through the phonon for high performance thermoelectric materials. Since the TiS6 layers are bonded by the weak van der Waals force, the thermal conductivity is inherently low. The atoms, which were intercalated into the boundaries between the TiS6 layers, not only participate in the carrier conduction but also serve as the phonon scattering centers. This results in reduction in thermal conductivity.
As described hereinbefore in detail, the compounds expressed by the general formula AxTiS2xe2x88x92y where 0xe2x89xa6xxe2x89xa62 and 0xe2x89xa6y less than 1 exhibit the low electrical resistivity, low thermal conductivity and large Seebeck coefficient by virtue of the above-described phenomena. As a result, large figure of merit ZT, i.e., S2T/xcfx81xcexa is achieved by the compounds, and are preferable for high conversion efficiency thermoelectric elements.
The thermoelectric materials according to the present invention are produced through a process for producing ceramic such as, for example, a firing on blended powder sealed in a quartz tube. Single crystal thermoelectric material according to the present invention is obtained through a precipitation from a high temperature molten compound, zone melting or chemical transportation. A thin film of the thermoelectric material according to the present invention is formed by using a sputtering, laser light evaporation or vacuum evaporation. The metals at the A-site may be intercalated into the lattice structure through an electrochemical process or reaction with organic compound.
Thermoelectric Generator and Thermoelectric Refrigerator
The thermoelectric materials according to the present invention are available for any kind of thermoelectric generator and any kind of thermoelectric refrigerator.